Express: scalable way to handle errors

Error handling is an important concept to understand for writing any better application. However often, I have seen error handling is misused or handled wrong way in many different application, especially in express.js application.

In this article, we'll talk about a way to handle the error in a better and scalable way.

Let’s build an user registration API with all functionality like validating user input, handling business validation, save the user's registration etc..,


app.post(
  "api/user/registration",
  ...
);

We will demonstrate 3 approaches such as novice, advanced and pro version of user's registration api.

Novice approach

In general we will come up with below code snippet without any proper design in place or this may be our first rough cut to build a registration endpoint.


app.post("api/user/registration", postRegistration);

function postRegistration(req, res, next) {
  const {
    first_name,
    last_name,
    email,
    password,
    re_password,
    terms_condition,
  } = req.body;

  const errors = [];

  // Required field validation
  if (!first_name) {
    errors.push("first_name is required");
  }
  if (!last_name) {
    errors.push("last_name is required");
  }
  if (!email) {
    errors.push("email is required");
  }
  if (!email) {
    errors.push("email is required");
  }
  if (!password) {
    errors.push("password is required");
  }
  if (!re_password) {
    errors.push("re_password is required");
  }
  if (!terms_condition) {
    errors.push("terms_condition is required");
  }

  // Length validation
  if (password.length > 8) {
    errors.push("Password has to be longer than 8 characters");
  }

  // cross field match validation
  if (password === re_password) {
    errors.push("Password and re_password has to match");
  }

  // Bad request error
  if (errors.length > 0) {
    return res.status(400).json({ errors: errors });
  }

  // Server business logic validation
  const businessErrors = [];

  if (email.includes("dummy@gmail.com")) {
    businessErrors.push("EMAIL_ALREADY_TAKEN");
  }

  if (password.includes("qwerty")) {
    businessErrors.push("AUTH_WEAK_PASSWORD");
  }

  if (businessErrors.length > 0) {
    return res.status(400).json({ businessErrors: businessErrors });
  }

  // await UserRegistrationRepo.register(req.body)
  res.json({ error: false, msg: "Registration is successful" });
}

Let's try to find the problem with above approach. It's easy to find many problems at a very first glance like code duplication, too many responsibilities for postRegistration function because it does multiple thing such as validating the input, handle business validation and handling database operation etc..,

Moving on to advanced version.

Advanced

In this version we will try to rectify the problem that we encountered in novice approach by eliminating the code duplication, split into responsibility, clean logical separations etc..,

app.post(
  "api/user/registration",
  validateRegistrationInput,
  validateBusinessRegistration,
  postRegistration
);

To avoid code duplication we have created our own util function to validate the different rules such as required, minLength etc..,

lib/util/validation.js

export function validate(input, validationRule) {
  return Object.keys(validationRule).reduce((errors, key) => {
    const currentRule = validationRule[key];
    if (currentRule.required) {
      if (!input[key]) {
        errors.push(`${key} is required field`);
      }
    }

    if (currentRule.minLength) {
      console.log({ errors, key, currentRule, input });
      if (input[key] && input[key].length < currentRule.minLength) {
        errors.push(
          `${key} has to more than ${currentRule.minLength} characters`
        );
      }
    }
    //TODO:cross field match validation
    return errors;
  }, []);
}

controller/registration.js

Let's see how our registration's controller code looks like

import { validate } from './validation'

const validationRule = {
  first_name: {
    required: true,
  },
  last_name: {
    required: true,
  },
  email: {
    required: true,
  },
  password: {
    required: true,
    minLength: 8,
  },
  re_password: {
    required: true,
    ref: "password",
    exactMatch: true,
  },
  terms_condition: {
    required: true,
  },
};

export function validateRegistrationInput(req, res, next) {
  const {
    first_name,
    last_name,
    email,
    password,
    re_password,
    terms_condition,
  } = req.body;

  const errors = validate(req.body, validationRule);

  // Bad request error
  if (errors.length > 0) {
    return res.status(400).json({ errors: errors });
  }
  next();
}

export function validateBusinessRegistration(req, res, next) {
  // Server business logic validation
  const { email, password } = req.body;
  const businessErrors = [];

  if (email.includes("dummy@gmail.com")) {
    businessErrors.push("EMAIL_ALREADY_TAKEN");
  }

  if (password.includes("qwerty")) {
    businessErrors.push("AUTH_WEAK_PASSWORD");
  }

  if (businessErrors.length > 0) {
    return res.status(400).json({ errorMessages: businessErrors });
  }

  next();
}

export function postRegistration(req, res, next) {
  // await UserRegistrationRepo.register(req.body)
  res.json({ success: true, data: { message: "Registration is successful" }});
}

Let's discuss some of the pros & cons of this advanced version of api/user/registration

Pros:

Reduced code duplication
clean separation
adhered single responsibility

Cons:

Not leveraging centralize error handling
Own implementation of validation (time-consuming to implement and test case to cover the all use-case)
Inconsistent error schema structure (How to provide consistent error schema to client ?)
- res.status(400).json({ errorMessages: businessErrors });
- res.status(400).json({ errors: errors });
Inconsistent response schema structure (How to provide consistent response schema to client ?)
- res.json({ success: true, data: { message: "Registration is successful" } });
- res.json({ error: false, msg: "Registration is successful" });

Just want to more emphasis on consistency, because consistency will lead a better, clean and understandable code. Also, which help us to refactor the code though code is messy.

Pro: Scalable Error handling

In pro version, we will take care of following things

Validation with Schema library (Yup/Joi)
Unified AppilcationError interface
Custom Error creation
Central error handling

Validation with Schema library

I'd like to introduce the schema based validation library such as Yup/Joi . Let's define the validation schema for our registration endpoint like below.

Take a look at our userRegistrationSchema function. See how elegantly we are validating against our javascript object without much code to write and also think about readablity perceptive, it has improved a lot and schema based validation is help us to reduce bug too!.

validation.js

import * as Yup from "yup";

export function userRegistrationSchema() {
  return Yup.object().shape({
    first_name: Yup.string().required(),
    last_name: Yup.string().required(),
    email: Yup.string().email().required(),
    password: Yup.string()
      .min(8, "Password has to be longer than 8 characters!")
      .required(),
    re_password: Yup.string()
      .oneOf([Yup.ref("password"), null], "Passwords must match")
      .required("Re-enter password is a required field"),
    terms_condition: Yup.boolean().oneOf(
      [true],
      "Please accept terms and conditions"
    ),
  });
}

Unified Appilcation Error interface

Let's create our own error class for whole application so that it will provide consistent error interface, error schema structure to the application and clients.

In other way, we may prefer to create our own custom error class which extends javascript's Error class, like below

class ResourceNotFound extend Error { }

It's up to us to decide but I felt too many error classes which bring some maintenance, enforce the consistency for error classes seems like it is unnecessary for javascript application. For example, In node.js internally errors are categorized into few type of errors.

Let's define ApplicationError class

lib/api/applicationError.js

export class ApplicationError extends Error {
  static type = {
    APP_NAME: "APP_NAME",
    INTERNAL: "INTERNAL",
    NETWORK: "NETWORK",
    UNKNOWN: "UNKNOWN",
  };

  constructor(options, overrides) {
    super();
    Object.assign(options, overrides);

    if (!ApplicationError.type.hasOwnProperty(options.type)) {
      throw new Error(`ApplicationError: ${options.type} is not a valid type.`);
    }

    if (!options.message) {
      throw new Error("ApplicationError: error message required.");
    }

    if (!options.code) {
      throw new Error("ApplicationError: error code required.");
    }

    this.name = "ApplicationError";
    this.type = options.type;
    this.code = options.code;
    this.message = options.message;
    this.errors = options.errors;
    this.meta = options.meta;
    // {
    //   analytics:  {},
    //   context: {}
    // }
    this.statusCode = options.statusCode;
  }
}

Great, now we have defined ApplicationError but think a moment about Yup's ValidationError and ApplicationError both are completely different interface.

How do we provide a consistent error interface ?

Since we are handling the third party exceptions like Yup validation or MongoException, which has the different error schema that will create a problem. We could solve this problem elegantly with the help of factory function. so that we can swap the Yup with Joi or something in later point even without knowing or altering too much in existing code.

Our Factory function name called createError that will take care of converting third party exception or any error to ApplicationError exception. Here the errorFactory.js appears below

lib/api/errorFactory.js

import * as Yup from 'yup'
import { ApplicationError } from './applicationError'

export function createError(error, overrides) {
  const isYupError = error instanceof Yup.ValidationError
  if (isYupError) {
    const yupError = mapYupValidationError(error)
    return new ApplicationError(yupError, overrides)
  }
  return new ApplicationError(error, overrides)
}

function mapYupValidationError(error) {

  return {
    type: ApplicationError.type.APP_NAME,
    code: 'VALIDATION_ERROR',
    message: error.message,
    errors: error.inner,
    statusCode: 400,
    meta: {
      context: error.value
    }
  }
}

Custom Error Creation

Return to our registration API, We might encounter the few business exceptions while developing the registration endpoint. Those few exceptions are

if email is already taken (EMAIL_ALREADY_TAKEN)
if user enters a weak password (AUTH_WEAK_PASSWORD)
...

As said, we don't want to create new custom error class for each type of error. then how do we create a custom error with the help of ApplicationError ?

controller/registration/error.js


import { ApplicationError } from '../../lib/api'

export const Errors = {
  EMAIL_ALREADY_TAKEN: {
    type: ApplicationError.type.APP_NAME,
    code: 'EMAIL_ALREADY_TAKEN',
    message: 'The given email address is already taken :(',
    statusCode: 400
  },
  AUTH_WEAK_PASSWORD: {
    type: ApplicationError.type.APP_NAME,
    code: 'AUTH_WEAK_PASSWORD',
    message: 'The given password is easy to guess, provide strong password',
    statusCode: 400
  }
}

In later we could use like below

new ApplicationError(RegistrationError.EMAIL_ALREADY_TAKEN);

one important thing to note, these business validation error.js are co-locating with our registration's controller is good thing.

Bonus: Common Errors

I'd like to show some of common errors which helps for REST API development.

lib/api/commonError.js

import { ApplicationError } from "./applicationError";

const HTTPError = {
  // Predefined 4xx http errors
  BAD_REQUEST: {
    type: ApplicationError.type.NETWORK,
    code: "BAD_REQUEST",
    message: "Bad request",
    statusCode: 400,
  },
  UNAUTHORIZED: {
    type: ApplicationError.type.NETWORK,
    code: "UNAUTHORIZED",
    message: "Unauthorized",
    statusCode: 401,
  },
  FORBIDDEN: {
    type: ApplicationError.type.NETWORK,
    code: "FORBIDDEN",
    message: "Forbidden",
    statusCode: 403,
  },
  RESOURCE_NOT_FOUND: {
    type: ApplicationError.type.NETWORK,
    code: "RESOURCE_NOT_FOUND",
    message: "Resource not found",
    statusCode: 404,
    meta: {
      translationKey: "app.common.error.RESOURCE_NOT_FOUND",
    },
  },

  // Predefined 5xx http errors
  INTERNAL_SERVER_ERROR: {
    type: ApplicationError.type.NETWORK,
    code: "INTERNAL_SERVER_ERROR",
    message: "Something went wrong, Please try again later.",
    statusCode: 500,
    meta: {
      shouldRedirect: true,
    },
  },
  BAD_GATEWAY: {
    type: ApplicationError.type.NETWORK,
    code: "BAD_GATEWAY",
    message: "Bad gateway",
    statusCode: 502,
  },
  SERVICE_UNAVAILABLE: {
    type: ApplicationError.type.NETWORK,
    code: "SERVICE_UNAVAILABLE",
    message: "Service unavailable",
    statusCode: 503,
  },
  GATEWAY_TIMEOUT: {
    type: ApplicationError.type.NETWORK,
    code: "GATEWAY_TIMEOUT",
    message: "Gateway timeout",
    statusCode: 504,
  },
};

export { HTTPError };

Bonus: Response Schema

In order to send the consistent response schema to the client we may need to define a function, called sendResponse, so that we can enforce to use sendResponse instead of res.json()

import { ApplicationError, createError } from '../error'

export function formatError(error, overrides = {}) {
  // `Error.stack`'s `enumerable` property descriptor is `false`
  // Thus, `JSON.stringify(...)` doesn't enumerate over it.
  const stackTrace = JSON.stringify(error, ['stack'], 4) || {}
  const newError = JSON.parse(JSON.stringify(error))

  // No need to send to client
  newError.statusCode = undefined
  delete newError.meta

  return {
    error: {
      ...newError,
      stack: stackTrace.stack
    },
    success: false,
    ...overrides
  }
}

export function formatResponse(result, override = {}) {
  return {
    data: result,
    success: true,
    ...override
  }
}

export function sendResponse(res, payload, statusCode = 200, context = {}) {
  return res.status(statusCode).json(formatResponse(payload))
}

As you can see here, validation.js and error.js are co-locating to registration endpoint would be great again.

Central error handling

It's time to reveal the core technique of this article that is centralized error handling in express.js application.

Define error-handling middleware functions in the same way as other middleware functions, except error-handling functions have four arguments instead of three: (err, req, res, next)

we should define an error-handling middleware last, after other app.use() and routes calls.

app.use("/api", userRegistrationRouter);

app.use(errorHandler);

how it works

In general, if error occur in synchronous code inside route handlers and middleware require no extra work. If synchronous code throws an error, then Express will catch and process it.

The errors returned from asynchronous functions invoked by route handlers and middleware, you must pass them to the next(error) function, where Express will catch and process them.

Like below we need to throw the error or pass the error to express middleware

controller/registration.js

import { userRegistrationSchema } from "./validation";
import { createError, sendJson, ApplicationError } from "../../lib/api";
import { Errors } from "./error";

export async function validateUserRegistration(req, res, next) {
  try {
    await userRegistrationSchema().validate(req.body, { abortEarly: false });
  } catch (e) {
    return next(createError(e));
  }
  next();
}

export function validationBusinessRule(req, res, next) {
  const { email, password } = req.body;

  if (email.includes('dummy@gmail.com')) {
    throw new ApplicationError(Errors.EMAIL_ALREADY_TAKEN);
  }

  if (password.includes('qwerty')) {
    throw new ApplicationError(Errors.AUTH_WEAK_PASSWORD);
  }
  next()
}
export function postRegistration(req, res, next) {
  // await UserRegistrationRepo.register(req.body)
  sendJson(res, { message: "Registration is successful" });
}

validationBusinessRule we synchronously throwing error hence we need not to call like next(error) and validateUserRegistration asynchronously we are catching the error so pass like next(error) to catch the express middleware.

Here is our centralized error middleware looks like

lib/errorHandler.js

import { sendResponse, formatError, CommonError } from "../lib/api";

export function errorHandler(err, req, res, next) {
    const { analytics = {} } = err.meta || {};
  // logging for analytics
  console.log({ analytics });

  if (err instanceof ApplicationError) {
    const code = err.statusCode || 500
    return res.status(code).json(formatError(err))
  }

  if (err instanceof Error) {
    const newError = createError(err)
    const code = newError.statusCode || 500
    return res.status(code).json(formatError(newError))
  }

  const unknownError = new ApplicationError(CommonError.UNKNOWN_ERROR)

  return sendResponse(res, unknownError, statusCode);
}

Most importantly, we did not handling errors in every middleware, All error handling moves to centralized error middleware with aim that we have great opportunities to cover other scenario easily such as

Logging error details
Send details for analytics
Formatting the error for consist error schema

Finally to test our registration endpoint using cURL command like below

curl --location --request POST 'http://localhost:3000/api/user/registration' \
--header 'Content-Type: application/x-www-form-urlencoded' \
--data-urlencode 'first_name=raja' \
--data-urlencode 'last_name=jaganathan' \
--data-urlencode 'password=qwerty1234' \
--data-urlencode 're_password=qwerty1234' \
--data-urlencode 'email=dummy@gmail.com' | python -mjson.tool

{
    "error": {
        "name": "ApplicationError",
        "type": "APP_NAME",
        "code": "AUTH_WEAK_PASSWORD",
        "message": "The given password is easy to guess, provide strong password"
    },
    "success": false
}

That's it. Pretty neat right!!!

You can find the repo here 💌https://github.com/RajaJaganathan/express-error-handling

Thanks for reading!

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